Data selection

Previously, yellow and silver eel series have been analyzed by WGEEL through trend analysis (ICES 2021a). Here, we have chosen to concentrate on improving the trend analysis for the silver eel time series.

In the 2023 data call, 57 silver eel time series were available, located in 15 countries (Figure @ref(fig:mapsilverall)). The majority of these series are from Lithuania (9 series), Netherlands (7 series), United Kingdom (6 series) and France (6 series). Four time series (BI1S, BI4S, NSIS, PanS) are international trawling surveys. Data on these silver eel time series have been added to the WGEEL database in the past, but as these international series have no dedicated reporting country, their data have not been inserted into the database for recent years.

Map of available silver time series. Updated time series correspond to time series for which at least one value was provided for years the three last years

Map of available silver time series. Updated time series correspond to time series for which at least one value was provided for years the three last years

Summary of data availability in the silver eel time series, showing availability of series with at least 10 years of data; and the number of series that do or do not have available information on series quality ID, habitat type, sampling type, effort, gear type, restocking influence, and distance to sea.

Category

Available data

Missing data

Nb of series >=10 years

30

27

Nb of series with quality id

36

21

Nb of series with habitat data

57

0

Nb of series with sampling type

53

4

Nb of series with effort data

23

34

Nb of series with gear

56

1

Nb of series with restocking data

56

1

Nb of series with distance to sea

52

5

Summary of available silver eel series per country, including whether they were kept or dropped for the trend analysis. Habitat: C = coastal water, F = freshwater, MO = marine water (open sea), T = transitional water (according to WFD); gear: 226 = fyke nets, 227 =stow nets, 228 = barriers, fences, weirs, etc., 230 = traps, 234 = longlines, 242 = electric fishing, 245 = gear unknown; sampling type: 1 = commercial catch, 2 = commercial CPUE, 3 = scientific estimate, 4 = trapping all, 5 = trapping partial gear; quality id: 0 = series has ended and no future data will become available, 1 = good quality data; stocking: FALSE = no impacts of stocking, TRUE = impacts of stocking; and ecoregion: WM = Western Mediterranean Sea, BS = Baltic Sea, NS = Greater North Sea, IB = Bay of Biscay and the Iberian Coast, CS = Celtic Seas, AL = Aegean-Levantine Sea, CM = Ionian Sea and the Central Mediterranean Sea, AS = Adriatic Sea. For all, NA means not available (no information provided).

Summary of available silver eel series per country, including whether they were kept or dropped for the trend analysis. Habitat: C = coastal water, F = freshwater, MO = marine water (open sea), T = transitional water (according to WFD); gear: 226 = fyke nets, 227 =stow nets, 228 = barriers, fences, weirs, etc., 230 = traps, 234 = longlines, 242 = electric fishing, 245 = gear unknown; sampling type: 1 = commercial catch, 2 = commercial CPUE, 3 = scientific estimate, 4 = trapping all, 5 = trapping partial gear; quality id: 0 = series has ended and no future data will become available, 1 = good quality data; stocking: FALSE = no impacts of stocking, TRUE = impacts of stocking; and ecoregion: WM = Western Mediterranean Sea, BS = Baltic Sea, NS = Greater North Sea, IB = Bay of Biscay and the Iberian Coast, CS = Celtic Seas, AL = Aegean-Levantine Sea, CM = Ionian Sea and the Central Mediterranean Sea, AS = Adriatic Sea. For all, NA means not available (no information provided).

A summary of the type of data available in the available silver eel time series is given in Table @ref(tab:tablesummary) and Figure @ref(fig:figureseriessummary), and a detailed summary of all the series is presented in Annex x.x. From all available silver eel time series, 30 series have 10 or more years of available data. Only 23 of the 57 series have effort data submitted. Most silver eel series were collected in freshwaters via traps and fyke nets. In terms of sampling type, 14 series were from commercial catches, 5 series was reported as commercial CPUE, 10 were assigned as full trapping series, 18 as partial trapping series, and 26 were classified as scientific estimate, with 4 series missing this information. When it comes to the series quality ID, 38 series had a quality ID of 1, indicating good quality, 40 had a quality ID of 0, indicating that it had been previously decided that the series should not be used in an analysis (usually because there were not enough years of data), and 21 series were missing a series quality ID. For the next WGEEL meeting, these series quality IDs should be reconsidered, given the large amount of series without a series quality ID, and the fact that some series with a quality ID of 0 now have at least 10 years of data. Almost all series had information on the potential impacts of restocking, with 21 series classified as being influenced by restocking and 35 as not being influenced by restocking.

The decision was made to perform the trend analysis with only those silver eel time series that had at least 10 years of data available. After examining the 57 available silver eel time-series, only 28 were kept for the trend analysis (Table @ref(tab:tablekept). From the 29 time-series that were excluded, 2 were showing inconsistencies in data and 27 were excluded because they did not have 10 consecutive years of effort data (Table @ref(tab:tabledropped).

The 28 silver eel time series which were used in the analysis represented six ICES ecoregions (Figure @ref(fig:mapsilverkept)). The Western Mediterranean Sea and Aegean-Levantine Sea only had one time series. The most represented areas were the Greater North Sea with 8 time-series and the Baltic Sea with 7 time-series. 11 ecoregions (Adriatic Sea, Arctic Ocean, Azores, Barents Sea, Black Sea, Faroes, Greenland Sea, Icelandic Waters, Ionian Sea and the Central Mediterranean Sea, Norwegian Sea, Oceanic Northeast Atlantic) did not have silver eel time series which were used in the trend analysis. All coastal time series used in the analysis are located in the Baltic Sea ecoregion. Freshwater series are located across 4 different ecoregions (Baltic Sea, Bay of Biscay and the Iberian Coast, Celtic Seas, Greater North Sea) and the transitional series are located in 2 ecoregions (Aegean-Levantine Sea, Western Mediterranean Sea).

Available silver eel time series that were kept for the analysis. Listed is also whether the silver eel time-series data was first divided by provided effort data, before it was used in the analysis. A reason for dividing by effort is that the data in the series is effort-dependent. Reasons for not first dividing by effort data include for instance the provided data already being a CPUE index, or effort being constant (e.g. whole year full-river coverage).

Country

Series name

Years (n)

Series type

Divided by effort data

Comments

BI1S

16

Index

No

BI4S

20

Index

No

NSIS

22

Index

No

PanS

16

Index

No

NO

ImsaS

48

Constant effort

No

SE

NkaS

41

Index

No

SE

SosS

41

Index

No

DE

WarS

14

Model estimate

No

DK

RibS

22

Model estimate

No

NL

HVWS

10

Index

No

NL

NiWS

11

Index

No

IE

BurS

51

Constant effort

No

IE

KilS

23

Effort-dependent

Yes

GB

BaBS

17

Constant effort

No

GB

FowS

12

Constant effort

No

GB

GiBS

32

Constant effort

No

GB

LevS

21

Constant effort

No

GB

ShiS

20

Constant effort

No

FR

BreS

37

Effort-dependent

Yes

FR

FreS

27

Constant effort

No

FR

LoiS

34

Index

No

FR

SeNS

10

Constant effort

No

FR

SouS

10

Model estimate

No

ES

AlCS

68

Effort-dependent

No

Commercial catches, but assumed that all eel are caught.

ES

BidS

13

Constant effort

No

ES

NalS

11

Constant effort

No

ES

OriS

16

Index

No

GR

EamtS

11

Constant effort

No

Available silver eel time series that were dropped from the analysis, including the reason for being dropped.

Country

Series name

Years (n)

Drop reason

Comments

SE

KavlS

4

Number of years fewer than 10

FI

KotkS

6

Number of years fewer than 10

FI

VaakS

9

Number of years fewer than 10

LV

DaugS

4

Number of years fewer than 10

LV

LilS

3

Number of years fewer than 10

LT

AlauS

4

Number of years fewer than 10

Next year double check for available effort data on all similar Lithuanian series, and check if only silver eel are reported

LT

CurlS

5

Number of years fewer than 10

Next year double check for available effort data on all similar Lithuanian series, and check if only silver eel are reported

LT

KertS

4

Number of years fewer than 10

Next year double check for available effort data on all similar Lithuanian series, and check if only silver eel are reported

LT

KreS

1

Number of years fewer than 10

Next year double check for available effort data on all similar Lithuanian series, and check if only silver eel are reported

LT

LakS

2

Number of years fewer than 10

Next year double check for available effort data on all similar Lithuanian series, and check if only silver eel are reported

LT

RieS

1

Number of years fewer than 10

Next year double check for available effort data on all similar Lithuanian series, and check if only silver eel are reported

LT

RubS

2

Number of years fewer than 10

Next year double check for available effort data on all similar Lithuanian series, and check if only silver eel are reported

LT

SiesS

3

Number of years fewer than 10

Next year double check for available effort data on all similar Lithuanian series, and check if only silver eel are reported

LT

ZeiS

1

Number of years fewer than 10

Next year double check for available effort data on all similar Lithuanian series, and check if only silver eel are reported

NL

BRWS

8

Number of years fewer than 10

NL

DoijS

8

Number of years fewer than 10

NL

IjsS

8

Number of years fewer than 10

NL

NZKS

7

Number of years fewer than 10

NL

ZMaS

9

Number of years fewer than 10

GB

StrS

6

Number of years fewer than 10

FR

VilS

9

Number of years fewer than 10

PT

MinS

5

Number of years fewer than 10

PT

MonS

6

Number of years fewer than 10

IT

PobeS

1

Number of years fewer than 10

IT

PogoS

1

Number of years fewer than 10

IT

PolsS

1

Number of years fewer than 10

IT

TibeS

1

Number of years fewer than 10

GR

NorwS

10

Inconsistencies in data

Recent years reported as 0, but actually no information available

GR

WepeS

10

Inconsistencies in data

Recent years reported as 0, but actually no information available

Map of silver eel time-series used in the trend analysis, shown by ICES ecoregion and habitat.

Map of silver eel time-series used in the trend analysis, shown by ICES ecoregion and habitat.

Figure @ref(fig:figureseries) gives an overview of the reported CPUE data for each silver eel time series that was kept for the trend analysis. Among the kept series, the number of available CPUE data points was highest between 2010 and 2021, with the peak in 2013-2017 (Figure @ref(fig:figurenseries)). The majority of the kept time series did not have 2023 data ready at the time of writing this report.

CPUE data of those silver eel time series that were used in the trend analysis.

CPUE data of those silver eel time series that were used in the trend analysis.

Number of available datapoints from silver eel CPUE time-series over time, shown only for those time-series that were used in the trend analysis.

Number of available datapoints from silver eel CPUE time-series over time, shown only for those time-series that were used in the trend analysis.